Translation, the mRNA-directed synthesis of proteins, occurs in three distinct steps: initiation, elongation and termination. Translation initiation is a complex process in which the two ribosomal subunits and methionyl tRNA (met-tRNA) assemble on a properly aligned mRNA to commence chain elongation at the AUG initiation codon. The established scanning mechanism for initiation involves the formation of a ternary complex among eukaryotic initiation factor 2 (eIF2), GTP and met-tRNA. The ternary complex recruits the 40S ribosomal subunit to form the 43S pre-initiation complex. This complex recruits mRNA in cooperation with other initiation factors such as eukaryotic initiation factor 4E (eIF4E), which recognizes the 7-methyl-guanidine cap (m-7GTP cap) in an mRNA molecule and forms the 48S pre-initiation complex. Cap recognition facilitates the 43S complex entry at the 5′ end of a capped mRNA. Subsequently, this complex migrates linearly until it reaches the first AUG codon, where a 60S ribosomal subunit joins the complex, and the first peptide bond is formed (Pain (1996) Eur. J. Biochem., 236:747-771).
Several features of the mRNA structure influence the efficiency of its translation. These include the m-7GTP cap, the primary sequence surrounding the AUG codon and the length and secondary structure of the 5′ untranslated region (5′ UTR). Indeed, a moderately long, unstructured 5′ UTR with a low G and C base content seems to be optimal to ensure high translational efficiency. Surprisingly, sequence analysis of a large number of vertebrate cDNAs has shown that although most transcripts have features that ensure translational fidelity, many do not appear to be designed for efficient translation (Kozak (1991) J. Cell. Biol., 115:887-903). Many vertebrate mRNAs contain 5′ UTRs that are hundreds of nucleotides long with a remarkably high GC content, indicating that they are highly structured because G and C bases tend to form highly stable bonds. Because highly structured and stable 5′ UTRs are the major barrier to translation, mRNAs with stable secondary structure in their 5′ UTR are translated inefficiently and their translation is highly dependent on the activity of translation initiation factors.
mRNAs with complex, highly structured 5′ UTRs include a disproportionately high number of proto-oncogenes such as the G1 cyclins, transcription and growth factors, cytokines and other critical regulatory proteins. In contrast, mRNAs that encode globins, albumins, histones and other housekeeping proteins rarely have highly structured, GC-rich 5′ UTRs (Kozak (1994) Biochimie, 76; 815-21; Kozak (1999) Gene, 234:187-208). The fact that genes encoding for regulatory but not for housekeeping proteins frequently produce transcripts with highly structured 5′ UTRs indicates that extensive control of the expression of regulatory genes occurs at the level of translation. In other words, low efficiency of translation is a control mechanism which modulates the yield of proteins such as cyclins, mos, c-myc, VEGF, TNF, among others, that could be harmful if overproduced.